(1) Field of the Invention
This invention relates to a pneumatic radial tire having an improved durability in the running on bad road surfaces.
(2) Description of the Prior Art
Radial tires, particularly heavy duty tires for trucks, buses and the like using steel cords as a reinforcement for the belt, carcass and the like are used not only on fully-paved and well-conditioned good roads, for instance, a route for the exclusive use of motor vehicles such as expressway or the like, but also on running roads partly including bad roads with an inferior road surface, such as a construction road or the like. Particularly, in the latter case, the tires are apt to have abnormal shortening of their use life due to separation rupture of the tread rubber layer at the initial use stage (hereinafter abbreviated as TLB), cut damage with the extremely poor appearance at the last life stage and the like, making recapping impossible.
Among the above problems, the conventional counter-measure for the improvement of cut resistance, i.e. a method of making the end count of cords in the belt small to lower the rigidity of the belt indeed provides an effect upon the initial cut resistance, but frequently causes cut damages in the rubber between the cords due to the smaller end count at the last life stage. Also, attempts have been made to use high elongation cords with a strand construction in the outermost belt layer. However, since the surface strain applied per one filament in such a cord during running on the bad road is too large, the separation failure is caused near the boundary between rubber and cord and particularly the TLB is induced at the last wear life stage. Even if the TLB is intended to be suppressed by decreasing the end count of the high elongation cords, the cut resistance is deteriorated at the last life stage likewise in the above case, so that it can not be used as a base tire for recapping.